Trusted Auth Framework will not fallback on the fallback authenticator during form-based authentication

Currently, the authenticator only falls back on the configured fallback authenticator during #checkAuth().

As #authenticate() is not implemented in the XWikiTrustedAuthenticator, authentication will be handled by XWikiAuthServiceImpl, which is only checking for local authentication, without taking into consideration any configured fallback authenticator.

The solution for this issue is to simply call method #authenticate() of the configured fallback authenticator in the XWikiTrustedAuthenticator, as #authenticate() is only called after #checkAuth() returns a null XWikiUser.